Service manual

Group 59 Anti-Lock Brake System (Bendix) Design and Function
When the ECU senses that a wheel or set of wheels is
about to lock during braking, a signal is sent to the ap-
propriate solenoid modulator valve to intermittently open
and close it. This opening and closing occurs rapidly (as
many as 15 times per second) to reduce brake pressure
and ensure that the wheel returns to rotating while still
maintaining most of the brake capacity.
If the brakes are applied and the wheel speed sensors
detect an impending wheel lockup, the electronic control
unit (ECU) controller will immediately begin to compen-
sate using the ABS modulator(s) at the affected wheel(s).
The rear axle brakes are controlled independently from
the front axle brakes, so that brake application pressure
at an individual wheel is adjusted solely on the basis of
its behavior on the road surface on which it is traveling.
Although each steering axle brake is under the control of
an individual modulator, the ECU controller does not
treat these brakes independently. The ECU uses a modi-
fied individual control for the steering axle brakes. This is
done in order to minimize “steering wheel pull” in the
event each wheel is traveling on a different road surface,
such as ice close to the curb and a dry crown. Essen-
tially, the ECU controls the brake force difference
between the two brakes. The wheel on dry pavement is
initially given less braking force and is brought up to opti-
mum during the stop, while the wheel on ice attempts to
maintain optimum braking during the entire stop.
Traction Control System
Some vehicles are equipped with a Traction Control Sys-
tem (TCS or ATC) included with the ABS. The TCS
prevents the drive wheels from spinning on a slippery
surface (such as on ice or loose gravel), or when accel-
erating from a standstill. The TCS uses the ABS ECU for
control, but requires additional control valves to automat-
ically apply brake force to the spinning wheels to
increase traction.
Components
The Bendix ABS consists of the following components:
•
Tooth Wheel (4 or 6)
•
Magnetic Sensor (4 or 6)
•
Modulator Valve (4)
•
Electronic Control Unit (ECU)
•
Fuses and Relays
•
ABS Warning Lamp
•
Wheel Spin Indicating Lamp
•
Connectors
•
TCS Switch (if TCS is installed)
•
Traction Control Valve (if TCS is installed)
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